Raman study of the phase transitions sequence in pure WO3 at high temperature and in HxWO3 with variable hydrogen content

An extensive investigation of the temperature dependence of Raman spectra has been carried out on WO3 powders from room temperature to 800°C. In particular the orthorhombic-to-tetragonal phase transition occurring at about 740°C has been studied for the first time. The Raman active mode at 710 cm−1 of the orthorhombic phase disappears from the spectrum at temperature below the phase transition point and the Raman activity in the tetragonal phase results very low. A comparative study of hydrogenated tungsten bronzes HxWO3 (x≤0.23), where the same transition sequence is driven by an increase of the proton concentration from x=0 to 0.23, reveals similar behaviour of the high frequency Raman active modes, which are better resolved due to the weaker anharmonic interactions.